This invention relates to an optical transport system, in particular, an optical fiber transport system using a wavelength-division-multiplexing technology.
As data communications represented by the Internet become widespread, replacement of communication lines with optical ones (optical fibers) rapidly becomes widespread for access networks, metro networks, or core networks. In the optical fiber transport system, a wavelength-division-multiplexing (WDM) transmission technology for multiplexing a plurality of main signal wavelengths is introduced in order to realize high-capacity long-distance transmission.
Typical characteristics that serve as an index of transmission quality in the optical fiber transport system include optical signal-to-noise ratio (OSNR) characteristics defined by a ratio of an optical signal to an optical noise.
The OSNR characteristics depend on characteristics including at least one of a “characteristic parameter of a transmission device” (for example, an output light intensity of an optical transmitter, an amplifier gain of an optical repeater, optical losses in an optical multiplexer, an optical demultiplexer and/or an intensity of the optical noise generated in the optical repeater), a “loss in a transmission fiber between transmission devices” (for example, span loss in an optical fiber), and a “number of optical repeaters within the transmission device”. If the characteristic of the “characteristic parameter of the transmission device” or the “loss in the transmission fiber between the transmission devices” cannot be uniformly obtained among wavelengths (channels), a deviation (disparity) occurs in the OSNR characteristics between wavelength channels.
On the other hand, in order to realize the transmission quality required in a system using the wavelength-division-multiplexing transmission technology (wavelength-division-multiplexing transport system), a predetermined OSNR must be ensured in a receiver located in the system. Therefore, the “loss in the transmission fiber between the transmission devices” or the “number of optical repeaters within the transmission device” is defined with reference to the channel whose OSNR becomes minimal in the wavelength-division-multiplexing transport system.
In other words, a transmission distance of the wavelength-division-multiplexing transport system is limited by the channel that exhibits the minimum OSNR. Therefore, there is a demand for improvement of the channel whose OSNR becomes minimal in the wavelength-division-multiplexing transport system. In other words, it is demanded to suppress an OSNR deviation between channels.
As a method of suppressing the OSNR deviation between channels, there is proposed a pre-emphasis technology that has an optical level adjustment function capable of adjusting optical power on a wavelength-by-wavelength (channel-by-channel) basis on an upstream side of a flow of the optical signal within the wavelength-division-multiplexing transport system and an OSNR monitor function of monitoring an OSNR on a wavelength-by-wavelength (channel-by-channel) basis on a downstream side of the optical signal within the wavelength-division-multiplexing transport system.
In the pre-emphasis technology, the optical level adjustment function is feedback-controlled based on a value of the OSNR measured by the OSNR monitor function. In other words, when a wavelength-division-multiplexed signal is transmitted to the OSNR monitor function, the optical level adjustment function is feedback-controlled in the pre-emphasis technology so that the OSNR characteristics between channels of the wavelength-division-multiplexed signal become uniform.
As a technology using the pre-emphasis technology, for example, Japanese Patent Application Laid-open No. Hei 8-321824 discloses a technology in which: “a pre-emphasis controller resets pre-emphasis amount setting devices and transmits wavelength-division-multiplexed signals; an optical signal-to-noise ratio measuring device measures optical signal-to-optical noise ratios of the respective wavelength-division-multiplexed signals and feeds back the optical signal-to-optical noise ratios to the pre-emphasis controller; the pre-emphasis controller automatically sets a pre-emphasis amount of the pre-emphasis amount setting devices based on the above-mentioned information; a line monitor device detects which optical amplifier's performance is deteriorated; and the pre-emphasis controller automatically controls the pre-emphasis amount of the pre-emphasis amount setting devices based on the detected information”.
Further, as another technology using the pre-emphasis technology, for example, Japanese Patent Application Laid-open No. Hei 9-261205 discloses a technology for “providing a system including: a first terminal that outputs WDM signal light by wavelength-division-multiplexing signal light beams of a plurality of channels having mutually different wavelengths; a second terminal that receives the WDM signal light; and an optical transmission line laid between the first terminal and the second terminal, in which: the second terminal has a function of monitoring signal-to-noise ratios of the respective channels based on the received WDM signal light; the optical transmission line includes a first line for transmitting the WDM signal light from the first terminal to the second terminal and a second line for transmitting a supervisory control signal related to the monitored signal-to-noise ratios from the second terminal to the first terminal; and the first terminal includes means for receiving the supervisory control signal and controlling power of the respective signal light beams based on the supervisory control signal so that the signal-to-noise ratios of the respective channels at the second terminal become the same”.
The pre-emphasis technology for monitoring an OSNR and suppressing the OSNR deviation between channels needs to include the function of monitoring the OSNR. Therefore, as a result of using the pre-emphasis technology, there occurs a problem of causing an increase in the size of the device and an increase in costs for operating the device.
Methods of monitoring signal light power of the respective channels (wavelengths) include a power monitoring method of splitting the wavelength-division-multiplexed signal into respective wavelengths by an optical demultiplexing function of an optical band-pass filter, an arrayed waveguide grating (AWG), or the like and receiving light of the respective optical signals obtained by the splitting by using photodiodes (PDs).
To monitor the OSNR by those methods, in other words, a power ratio of the signal light power to noise light power, it is necessary to monitor the noise light power as well as the signal light power at the same time. Here, of noise light in the wavelength-division-multiplexing transport system, the noise light output from a light source and the noise light (amplified spontaneous emission (ASE)) output from an optical amplifier located in the transport system are dominant.
In the power monitoring method of receiving the optical signal split on a wavelength-by-wavelength basis by using the PDs by the optical demultiplexing function described above, it is impossible to split the noise light from the signal light or monitor the respective powers obtained by the splitting. Therefore, it is necessary to provide a function of improving a wavelength resolution of the above-mentioned optical demultiplexing function to an extent that allows the noise light to be split from the signal light and monitoring the respective optical powers of the signal light and the noise light.
A general function of monitoring the respective optical powers of the signal light and the noise light is realized by a device such as an optical spectrum analyzer. The optical spectrum analyzer is a device that measures the optical signal by sweeping an optical receiver and a diffraction grating located immediately in front thereof or the optical band-pass filter in a wavelength axis direction and splitting the noise light power from the signal light power with high wavelength resolution. Also in Japanese Patent Application Laid-open No. Hei 8-321824, the optical spectrum analyzer is used as an OSNR monitor.
However, the optical spectrum analyzer is realized by making the most of an advanced control technology for sweeping the diffraction grating or the optical band-pass filter in the wavelength axis direction with high resolution and high precision and an optical filter technology having steep and high-precision wavelength characteristics. Therefore, compared to the general power monitoring method of receiving the optical signal split on a wavelength-by-wavelength basis by using the PDs by the optical demultiplexing function, the optical spectrum analyzer is expensive and hard to integrate, and a device equipped with the optical spectrum analyzer leads to a problem of causing an increase in the costs of the device and an increase in the size of the device.
In addition, as an alternative method to the above-mentioned OSNR monitor, Japanese Patent Application Laid-open No. Hei 9-261205 discloses a method of monitoring an electrical SNR. In the monitoring of the electrical SNR, an SNR can be monitored in an equivalent manner by measuring a Q factor (quality factor). On the other hand, it is necessary to provide an advanced and high-speed signal processing circuit for performing a signal processing for calculating a statistical distribution of signal power levels after converting the optical signal into an electrical signal, a processing for estimating the transmission quality from a partial signal included in original information after decoding the original information from the signal, or other such processing. Therefore, the monitoring of the electrical SNR leads to a problem of causing an increase in the costs of the device.